KVM: x86: Add helpers to identify CTL and STATUS MCi MSRs

Add helpers to identify CTL (control) and STATUS MCi MSR types instead of
open coding the checks using the offset.  Using the offset is perfectly
safe, but unintuitive, as understanding what the code does requires
knowing that the offset calcuation will not affect the lower three bits.

Opportunistically comment the STATUS logic to save readers a trip to
Intel's SDM or AMD's APM to understand the "data != 0" check.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20220512222716.4112548-4-seanjc@google.com

arch/x86/kvm/x86.c

@@ -3191,6 +3191,16 @@ static void kvmclock_sync_fn(struct work_struct *work)
 					KVMCLOCK_SYNC_PERIOD);
 }
 
+/* These helpers are safe iff @msr is known to be an MCx bank MSR. */
+static bool is_mci_control_msr(u32 msr)
+{
+	return (msr & 3) == 0;
+}
+static bool is_mci_status_msr(u32 msr)
+{
+	return (msr & 3) == 1;
+}
+
 /*
  * On AMD, HWCR[McStatusWrEn] controls whether setting MCi_STATUS results in #GP.
  */
@@ -3228,9 +3238,6 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (msr > last_msr)
 			return 1;
 
-		offset = array_index_nospec(msr - MSR_IA32_MC0_CTL,
-					    last_msr + 1 - MSR_IA32_MC0_CTL);
-
 		/*
 		 * Only 0 or all 1s can be written to IA32_MCi_CTL, all other
 		 * values are architecturally undefined.  But, some Linux
@@ -3241,15 +3248,21 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		 * UNIXWARE clears bit 0 of MC1_CTL to ignore correctable,
 		 * single-bit ECC data errors.
 		 */
-		if ((offset & 0x3) == 0 &&
+		if (is_mci_control_msr(msr) &&
 		    data != 0 && (data | (1 << 10) | 1) != ~(u64)0)
 			return 1;
 
-		/* MCi_STATUS */
-		if (!msr_info->host_initiated && (offset & 0x3) == 1 &&
+		/*
+		 * All CPUs allow writing 0 to MCi_STATUS MSRs to clear the MSR.
+		 * AMD-based CPUs allow non-zero values, but if and only if
+		 * HWCR[McStatusWrEn] is set.
+		 */
+		if (!msr_info->host_initiated && is_mci_status_msr(msr) &&
 		    data != 0 && !can_set_mci_status(vcpu))
 			return 1;
 
+		offset = array_index_nospec(msr - MSR_IA32_MC0_CTL,
+					    last_msr + 1 - MSR_IA32_MC0_CTL);
 		vcpu->arch.mce_banks[offset] = data;
 		break;
 	default: